1. Field of the invention
The present invention relates to a ball lens assembly to be used in an optical device such as a light emitting device module or a light receiving device module.
2. Description of the Related Art
In the field of optical communication, a ball lens is widely used, so as to condense light emitted from a light emitting device or a light emitting end of an optical fiber and make the light enter a light entering end of an optical fiber, or so as to condense light emitted from the light emitting end of the optical fiber and make the light enter a light receiving device. The ball lens is not easy to fix and hold because it is spherical, and it is apt to break. Accordingly, these points must be considered in manufacturing a ball lens assembly.
It is known that the ball lens assembly is constructed by press-fitting the ball lens into a sectionally circular press-fit hole formed in a lens holder. In the use of such a ball lens assembly, a position of the ball lens relative to other optical parts such as a light emitting device can be decided by fixing the lens holder to a substrate or the like of an optical device.
In fixing the lens holder to the substrate, fixation by laser welding is effective so that aged displacement of the lens holder after fixed may be prevented. However, a metal material suitable for laser welding, such as stainless steel, is generally hard. Accordingly, in the case that the accuracy of interference (difference between an outer diameter of the ball lens and an inner diameter of the lens holder) is low, there is a possibility of the ball lens being broken. While the breakage of the ball lens during press-fitting thereof may be prevented by increasing the accuracy of the interference, it becomes not easy to manufacture the lens holder in this case, causing an increase in cost.
In view of the above problem, there has been proposed a structure of a ball lens assembly as shown in FIGS. 1A and 1B. This ball lens assembly is constructed of a composite lens holder and a ball lens 3 press-fitted in the composite lens holder. The composite lens holder is constructed of an outer cylindrical member 1 and an inner cylindrical member 2 fixed to the outer cylindrical member 1. The outer cylindrical member 1 is made of a weldable metal material such as stainless steel, and the inner cylindrical member 2 is made of a relatively soft metal material such as copper.
According to this ball lens assembly, laser welding to the substrate or any other metal parts is possible, and a force for press-fitting the ball lens into the composite lens holder can be suppressed even when the accuracy of the interference is low. However, this ball lens assembly is not easy to manufacture, still causing an increase in cost.